1. Field of the Invention
This invention relates to conductive polymer compositions and electrical devices comprising such compositions.
2. Introduction to the Invention
Conductive polymers and electrical devices comprising them are well-known. Conventional conductive polymer compositions comprise an organic polymer, often a crystalline organic polymer, and, dispersed in the polymer, a particulate conductive filler such as carbon black or metal particles. Reference may be made, for example, to U.S. Pat. Nos. 4,237,441 (van Konynenburg et al), 4,388,607 (Toy et al), 4,534,889 (van Konynenburg et al), 4,545,926 (Fouts et al), 4,560,498 (Horsma et al), 4,591,700 (Sopory), 4,724,417 (Au et al), 4,774,024 (Deep et al), 4,935,156 (van Konynenburg et al), and 5,049,850 (Evans et al), and copending, commonly assigned application Ser. Nos. 07/788,655 (Baigrie et al), filed Nov. 6, 1991, now U.S. Pat. No. 5,250,228, issued Oct. 5, 1993, and 07/894,119 (Chandler et al), filed Jun. 5, 1992. The disclosure of each of these patents and applications is incorporated herein by reference.
Many conductive polymer compositions exhibit positive temperature coefficient of resistance (PTC) behavior, i.e. the resistance increases anomalously from a low resistance, low temperature state to a high resistance, high temperature state at a particular temperature, i.e. the switching temperature T.sub.s. The ratio of the resistance at high temperature to the resistance at low temperature is the PTC anomaly height. When the composition is in the form of a circuit protection device placed in series with a load in an electrical circuit, under normal operating conditions the device has a relatively low resistance and low temperature. If, however, a fault occurs, e.g. due to excessive current in the circuit or a condition which induces excessive heat generation within the device, the device "trips", i.e. is converted to its high resistance, high temperature state. As a result, the current in the circuit is reduced and other components are protected. When the fault condition is removed, the device resets, i.e. returns to its low resistance, low temperature condition. Fault conditions may be the result of a short circuit, the introduction of additional power to the circuit, or overheating of the device by an external heat source, among other reasons. For many circuits, it is necessary that the device have a very low resistance in order to minimize the impact of the device on the total circuit resistance during normal circuit operation. As a result, it is desirable for the composition comprising the device to have a low resistivity, i.e. less than 10 ohm-cm, which allows preparation of relatively small, low resistance devices. In addition, for some applications, e.g. circuit protection of components in the engine compartment or other locations of automobiles, it is necessary that the composition be capable of withstanding ambient temperatures which are relatively high, e.g. as much as 125.degree. C. without changing substantially in resistivity. In order to successfully withstand such exposure, it is desirable that the melting point of the composition be higher than the expected ambient temperature. Among those polymers which have relatively high melting points are crystalline fluorinated polymers.
Crystalline fluorinated polymers, also referred to herein as fluoropolymers, have been disclosed for use in conductive polymer compositions. For example, Sopory (U.S. Pat. No. 4,591,700) discloses a mixture of two crystalline fluoropolymers for use in making relatively high resistivity compositions (i.e. at least 100 ohm-cm) for self-limiting strip heaters. The melting point of the second polymer is at least 50.degree. C. higher than that of the first fluoropolymer and the ratio of the first polymer to the second polymer is 1:3 to 3:1. Van Konynenburg et al (U.S. Pat. No. 5,093,898) discloses compositions for use in flexible strip heaters or circuit protection devices which are prepared from polyvinylidene fluorides which have a low head-to-head content (i.e. a relatively low number of units of --CH.sub.2 CF.sub.2 ----CF.sub.2 CH.sub.2 -- compared to --CH.sub.2 CF.sub.2 ----CH.sub.2 CF.sub.2 --). Lunk et al (U.S. Pat. No. 4,859,836) discloses a melt-shapeable composition in which a first fluoropolymer of relatively low crystallinity and a second fluoropolymer of relatively high crystallinity which is not melt-shapeable in the absence of other polymers, e.g. irradiated polytetrafluorethylene, are mixed to produce a highly crystalline material suitable for use in heaters and circuit protection devices. Chu et al (U.S. patent application Ser. No. 08/021,827, filed Feb. 24, 1993, now U.S. Pat. No. 5,317,061, issued May 31, 1994) discloses a mixture of a copolymer of tetrafluoroethylene and hexafluoropropylene (FEP), a copolymer of tetrafluoroethylene and perfluoropropylvinyl ether (PFA), and polytetrafluoroethylene to prepare a composition which has good physical properties and exhibits little stress-cracking when exposed to elevated temperatures. The disclosure of each of these patents and applications is incorporated herein by reference.